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<h2><span class="refentrytitle">Macro BOOST_VMD_IS_EMPTY</span></h2>
<p>BOOST_VMD_IS_EMPTY — Tests whether its input is empty or not. </p>
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<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="header/boost/vmd/is_empty_hpp.html" title="Header &lt;boost/vmd/is_empty.hpp&gt;">boost/vmd/is_empty.hpp</a>&gt;

</span>BOOST_VMD_IS_EMPTY(...)</pre></div>
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<a name="idm45306167753984"></a><h2>Description</h2>
<p>The macro checks to see if the input is empty or not. It returns 1 if the input is empty, else returns 0.</p>
<p>The macro is a variadic macro taking any input. For the VC++8 compiler (VS2005) the macro takes a single parameter of input to check.</p>
<p>For all levels of C++ prior to C++20 the macro is not perfect, and can not be so. The problem area is if the input to be checked is a function-like macro name, in which case either a compiler error can result or a false result can occur.</p>
<p>For C++20, with its support for the new <span class="bold"><strong>VA_OPT</strong></span> preprocessor construct, the macro will always work correctly no matter what the variadic input, and is therefore 100% reliable.</p>
<p>This macro is a replacement, using variadic macro support, for the undocumented macro BOOST_PP_IS_EMPTY in the Boost PP library. The code is taken from a posting by Paul Mensonides of a variadic version for BOOST_PP_IS_EMPTY, and changed in order to also support VC++. The code for the C++20 implementation of the macro, using the <span class="bold"><strong>VA_OPT</strong></span> preprocessor construct, is the author's own and reuses code added to the Boost preprocessor library by this author.</p>
<p>... = variadic input, for VC++8 this must be a single parameter</p>
<p>returns = 1 if the input is empty, 0 if it is not</p>
<p>It is recommended to append BOOST_PP_EMPTY() to whatever input is being tested in order to avoid possible warning messages from some compilers about no parameters being passed to the macro when the input is truly empty. </p>
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